The present invention concerns a component part, and in particular to an acoustically effective rear shelf, or a cover for wheel guards, for storage compartments or luggage compartments in vehicles, as well as to a method for producing the same.
Rear shelves are important components in the interior design of motor vehicles. They serve not only as a place to deposit articles, but they also separate the luggage compartment from the passenger compartment. It is therefore necessary that they are aesthetically pleasing, that they are sufficiently rigid to support weights of up to 30 kg, and, in particular, that they are acoustically effective in order to dampen the sound field transmitted from the luggage compartment to the passenger compartment, for example by the wheel guards. Furthermore these rear shelves must be light weight, economical to manufacture and must be able to meet the customer-specific requirements for vehicle interior linings.
A number of different interior linings are known which comprise a stiffening, honeycomb-like support member having reinforcing fiber layers on both sides and having at least one decor layer. Such a component is known, for example, from EP 0 787 578. This publication describes a multilayered component, whose support member has a honeycomb structure and whose cover layers are connected at the front edges of the individual honeycomb cells by frictional or form contact. These cover layers are made of a thermoplastic synthetic material and are not permeable to air. This component has a sound damping effect and is not suitable for use as an acoustically effective rear shelf in vehicles.
It is the aim of the present invention to provide a rear shelf having a high sound absorption, which is light weight, sufficiently rigid and is simple to manufacture. It is a further aim of the present invention to provide a component having distinct damping characteristics in order to controllably dampen the resonance emitted from luggage compartments, spare wheel guards and other storage spaces.
The publication WO 99135007 describes an ultralight, sound and shock absorbing assembly. This assembly comprises an intermediate layer 3 formed of a plurality of hollow or tube-like elements 2. These tube-like elements 2 fulfil two functions, namely they determine the shock absorbing capacity of the whole assembly on the one hand, and they serve the acoustic absorbing capacity on the other hand. However, this is only possible because a covering layer 6 functions as a sound permeable covering layer and thereby allows the field of sound which is to be absorbed to interact with the labyrinth of hollow spaces formed by the tube-like elements. This sound and shock absorbing assembly is therefore not suitable for use as a rear parcel shelf because of the loosely positioned layers.
In EP 0 658 644 a lightweight component part is described which is made of textiles having a large elasticity module. This lightweight component part is especially suitable for the production of overhead baggage compartments, as they are used by the airplane industry. These component parts do not have a particular acoustical absorption capacity and are therefore not suitable for use as acoustically effective rear parcel shelves in motor vehicles.
These aims are achieved by a component having honeycomb-like core layer, which is provided on both sides with a reinforcing layer. These reinforcing layers consist of an air permeable and thermoplastic fibrous material which comprises, for example, at least 50% polypropylene fibers. In this context, polypropylene fibers are intended to mean fibers made entirely of polypropylene or which are co-extruded, having a coating of polypropylene and a core of, for example, polyester which melts at higher temperatures. In a preferred embodiment of the inventive rear shelf the polypropylene fibers are materially bonded to or fused with the core layer. The air-permeability of the reinforcing layers is essential for the acoustic performance of the inventive rear shelf, whereby the air flow resistance of these reinforcing layers can be adjusted for a desired range. The choice and composition of the fibers/bonding agents used lies within the expertise of the specialist. It is therefore clear that these reinforcing layers can be manufactured using additives made of powdered bonding agents and/or semi-thermoplastic lacquer powder. In a preferred embodiment the proportion of reinforcing fibers contained in the reinforcing layers is around 30%. Mineral fibers, preferably glass fibers, synthetic fibers, preferably PES- or PA-fibers, polyacrylic, aramide and other fibers known to the specialist can be used as reinforcing fibers. The reinforcing layer on the passenger compartment side comprises between 50% to 80%, and preferably about 70% polypropylene fibers and between 20% to 50%, and preferably about 30% glass fibers or polyester fibers or aramide fibers. These reinforcing layers preferably have an area weight of between 300 to 1000 g/m2 and are particularly suitable for the inventive rear shelf. In a further embodiment, the reinforcing layer comprises a first fibrous nonwoven layer having a polypropylene fiber content of approx. 20% and a second connecting layer with a light area weight of around 20-50 g/m2 and composed of SMMS-fibers (Spun-bond, Melt-blown, Melt-blown, Spun-bond polypropylene fibers). It is to be understood that the thickness of the core layer can be varied according to the requirements. The honeycomb like core layer has a thickness of 10 mm to 30 mm and an area weight in the range of 800 g/m2 to 1600 g/m2, that means has a density of 25 kg/M3 to 160 kg/M3.
This aim is further achieved by a method whereby a stack having at least one honeycomb-like core layer with precompressed reinforcing layers arranged on both sides thereof is arranged between two heatable plates. With the aid of these plates the stack is heated such that the thermoplastic bonding fibers of the reinforcing layers and the bonding region of the core layer are softened and begin to melt, i.e. the core layer retains its shape and the reinforcing layers retain their air-permeability. After this heating phase the stack is transferred into a cold (moulding) tool, where the individual layers are bonded and are brought into their final form.